The Book Pertaining to the Merits of the Companions (Allah Be Pleased With Them) of the Holy Prophet (May Peace Be Upon Him) (Kitab Al-Fada'il Al-Sahabah)
Muslim :: Book 31 : Hadith 6093
Abu Musa reported that Allah's Messenger (may peace be upon him) said: I recognise the voice of the Ash'arites while they recite the Qur'an as they arrive during the night and I also recognise their station from the recital of the Qur'an during the night time, although I have not seen their encampments as they encamp during the day time. And there is a person amongst them, Hakim; when he encounters the horsemen or the enemies he says to them: My friends command you to wait for them.